The present invention relates to an image forming apparatus, and more particularly to an automatic adjustment control of a pulse width modulation circuit for modulating a laser beam corresponding to an image signal.
Conventionally, an image forming apparatus has been widely known in which a laser beam is modulated corresponding to an image signal, and in which the modulated laser beam scans the surface of a recording medium so that gradation can be reproduced (refer to Japanese Patent Publication Open to Public Inspection No. 39974/1987).
The pulse width modulation is conducted as follows: for example, a digital image signal is converted to an analog signal (DC voltage signal); the converted analog signal is compared to, for example, a pattern signal as shown in FIG. 8; and a pulse width modulation signal SPWM having a pulse width corresponding to an image signal can be obtained.
A packaged IC having the a pulse width modulation circuit in one package is currently produced, and some of the pulse width modulation ICs have an automatic adjustment function. The automatic adjustment function is performed as follows: a calibration signal, inputted from the outside of the apparatus, is used as a trigger signal; and in a full scale range of the pulse width modulation signal SPWM, a reference voltage for generating a pattern signal, a reference voltage of a D/A converter, and the like are automatically set.
Here, because input-output characteristics of the pulse width modulation IC vary mainly due to temperature differences, generally, the calibration signal is generated by the structure shown in FIG. 9.
A pulse width modulation IC 51 shown in FIG. 9 is operated as follows: a dot clock DCK and an 8 bit image data DATA are sent to the IC; one of three pulse width modulation modes of the IC is selected by control signals A and B (refer to FIG. 5); and the IC outputs a pulse width modulation signal PWMOUT according to the selected pulse width modulation mode.
In this situation, a temperature sensor 52 for detecting the temperature of the pulse width modulation IC51 is provided; and a voltage outputted from the temperature sensor 52 corresponding to a temperature is converted by an A/D converter 53, and outputted to a CPU 54.
In the CPU 54, the following operations are conducted: a temperature variation is found when current temperature data is compared to the temperature data at the time of the previous calibration; then, in the case where the temperature variation is more than an absolute value of, for example, .+-.10.degree. C., when a page area signal indicating an image formation area is low, and no image data DATA exists, then a temperature variation detection signal showing the temperature variation is outputted to a calibration signal generator 55.
On the other hand, under the condition that the page area signal is high and the image data DATA exists, the temperature variation detection signal is outputted to the calibration signal generator 55 after processing of the image data DATA has been completed, that is, after the page area signal has been changed to low.
In the calibration signal generator 55, when the temperature variation detection signal is inputted, a calibration signal is generated, and the signal is outputted to a pulse width modulation IC 51. Then, in the pulse width modulation IC 51 when the calibration signal is inputted to the IC 51, an initializing operation is conducted and a full scale range of the pulse width modulation is set.
In this connection, when the calibration signal is generated by the aforementioned circuit composition, the following problems occur: a temperature sensor and an A/D converter are necessary, resulting in a complex circuit composition; and it is necessary to calculate a judgement data in order to judge the temperature variation, and thereby, the calculation load of the CPU is increased.
Further, when the temperature of the pulse width modulation IC is not accurately detected, the automatic adjustment function is inaccurately operated. Accordingly, a highly accurate and expensive temperature sensor is necessary. Another problem is that this composition can not flexibly meet other requirements for automatic adjustment which occur from conditions other than the temperature variation.